1. Field of the Invention
The present invention relates to an active matrix liquid crystal display device, and more particularly, to an active matrix liquid crystal display device which has a higher contrast and which can be driven by a lower voltage.
2. Description of the Prior Art
As shown in FIG. 1 and FIG. 2, a conventional active matrix liquid crystal display device has a general constitution comprising a thin film transistor array substrate 150, a transparent opposite substrate (glass substrate) 140 disposed parallel to and spaced from the thin film transistor array substrate 150, and a liquid crystal layer 130 sealed between the thin film transistor array substrate 150 and the opposite substrate 140. Note, FIG. 1 is a plan view showing a unit pixel of the thin film transistor array substrate 150 under the liquid crystal layer 130 of the active matrix liquid crystal display device shown in FIG. 2, while FIG. 2 is a cross-sectional view along a line Xxe2x80x94X of FIG. 1 so as to also show those parts above the thin film transistor array substrate 150.
In FIG. 1, the thin film transistor (TFT) is constituted of a gate electrode 1, an amorphous silicon film 119, a drain electrode 3 and a source electrode 4. The gate electrode 1 is electrically connected to a scan line 11 at the applicable stage, the drain electrode 3 is electrically connected to a signal line 12 at this column, and the source electrode 4 is electrically connected to a pixel electrode 5. Reference numeral 13 designates an adjacent signal line, so that a unit pixel is formed in an area surrounded by the scan line 11, a scan line 101 at a preceding stage, and the signal lines 12 and 13.
In a liquid crystal display device having such a constitution when the TFT is turned on for each matrix segment, an electric field is generated between the pixel electrode 5 and an opposite electrode 123 so that the liquid crystal layer 130 sealed between both substrates 100, 140 exhibits an electro-optic effect, such that the panel can display an image as a whole.
FIG. 3 is a diagram showing a gate-source parasitic capacitance of a TFT. As shown in FIG. 3, in an active matrix liquid crystal display device utilizing a TFT, there is generated a gate-source parasitic capacitance Cgs in an area where the gate electrode 1, source electrode 4 and drain electrode 3 overlap with each other.
FIG. 4 is a diagram showing an equivalent circuit of one pixel of a conventional liquid crystal display device provided with a TFT element. In FIG. 4, Cgs is the parasitic capacitance between the gate electrode 1 and source electrode 4 of the TFT, CLC is a capacity of the liquid crystal layer 130 between the pixel electrode 5 and the opposite electrode 123, and CSC is an accumulation capacity to be formed between the pixel electrode 5 and the scan line 101.
FIG. 5 is a diagram showing a voltage waveform for driving the aforementioned liquid crystal display device, in which the pixel electrode 5 is gradually accumulated with an electric charge such that the pixel electrode potential approaches a potential of the signal line, when the potential of the gate electrode 1 is high. Then, when the gate potential is turned off, the potential of the pixel electrode 5 exhibits a voltage drop, since this potential is pulled toward minus by the gate potential via parasitic capacitance Cgs. This dropped amount xcex94V is called a feedthrough voltage.
Paying attention to one piece of displaying pixel, it is common to drive a liquid crystal display device by applying an AC voltage having an alternating polarity for each display frame, between the opposite electrode 123 and the pixel electrode 5 so as to ensure the reliability of the device. The parasitic capacitance Cgs of a TFT can be regarded as an MIS (Metal-Insulator-Semiconductor) capacitor, and from a qualitative analysis the present applicant has found that the effective value of the MIS capacity at a positive writing differs from that at a negative writing, causing that the magnitudes of the feedthrough voltage xcex94V at a positive writing and a negative writing are different from each other, and the magnitude of xcex94V at the negative writing becomes larger. The details of the above will be fully discussed in the detailed description of the embodiments of the present invention, and will be only briefly discussed here.
Namely, considering that the voltage to be applied to a liquid crystal layer is a difference (xcex94VPI of FIG. 5) between a common electrode potential and a pixel electrode potential, and considering that in a liquid crystal display device a positive writing and a negative writing are generally switched to each other at each frame so as to ensure the reliability; as the feedthrough voltage at the negative writing is larger than the feedthrough voltage at the positive writing, the voltage to be applied to the liquid crystal layer at the time (unselected period) other than the selected interval (the time during which the gate potential is high) becomes larger (i.e., xcex94VPI becomes larger). It has been also qualitatively found that the larger the absolute value of Cgs, the larger the difference between feedthrough voltages at the positive writing and the negative writing (however, the ratio is substantially constant).
The present applicant has noticed that this phenomenon suggests a possibility that there is exhibited an effect capable of allowing a liquid crystal layer to be applied with a voltage larger than a voltage applied from the exterior, such as by increasing a TFT area or by disposing a plurality of TFT""s, i.e., by increasing an MIS capacity to be coupled to the scan line 11 and the pixel electrode 5. However, when a TFT area is increased, there is generally caused such a problem of a reduction of a numerical aperture as well as an electric charge leakage at the off time. Further, when a large delay has occurred in a gate pulse, a considerable electric charge will flow into the pixel electrode until the TFT becomes off, so that the aforementioned voltage amplification effect is reduced.
The present invention has been carried out in view of the problems as described above, and it is therefore an object of the present invention to provide an improved active matrix liquid crystal display device which has a higher contrast and which can be driven by a lower voltage, while avoiding occurrence of such problems of a reduction of a numerical aperture, a retention property at the off time of the TFT, and a reduction of a voltage amplification effect at the time of delay of a gate pulse, to be considered in a conventional active matrix liquid crystal display device.
The present invention provides, as a solving means for the aforementioned object, an active matrix liquid crystal display device having a thin film transistor array substrate comprising a plurality of scan lines formed on an insulating substrate, a plurality of signal lines formed to cross the scan lines, thin film transistors formed near intersections between the scan lines and signal lines, respectively, and additional capacity portions connected to pixel electrodes connected to source electrodes of the thin film transistors, respectively, in which a part of the additional capacity portion is formed, via insulating film and semiconductor, between the pixel electrode and that scan line which is connected to a gate electrode of the thin film transistor for switching the pixel electrode.
In the active matrix liquid crystal display device having the above constitution, a part of the disposing area of the semiconductor provided between the pixel electrode and the scan line preferably overlaps with the intersection between the signal line and the scan line.
In the active matrix liquid crystal display device having the above constitution, the semiconductors are preferably formed into a string shape on the scan line.
In the active matrix liquid crystal display device having the above constitution, the pixel electrode is preferably overlapped with the scan line at the preceding stage at a portion where the semiconductor at the preceding stage is not formed.
In the active matrix liquid crystal display device having any of the above constitutions, the semiconductor disposed between the pixel electrode and the scan line is preferably constituted of an amorphous silicon film.
In the active matrix liquid crystal display device having any of the above constitutions, the semiconductor disposed between the pixel electrode and the scan line may be constituted of a polycrystal silicon film.